1. Field of Invention
The present invention relates to electrophotographic apparatus and more particularly to improved corona charging structure for in-place primary charging of the photoconductive image member.
2. Description of Prior Art
In electrophotographic imaging an overall primary electrostatic charge is applied to the photoconductive imaging member prior to its imagewise exposure. This primary charge should be uniform at different loci within a given imaging area (i.e., have intra-image charge uniformity) in order to achieve uniformity of tone in the final electrographic image, after exposure and development. Also, the level of the primary charge should be consistent for successive image areas because inter-image variations from a nominal level create overall image inconsistencies, e.g., images which are too light or too dark throughout their respective image area.
The uniformity and consistency of primary charge also is important to other aspects of electrophotographic imaging; and there has been continual effort directed toward the development of corona charging devices which will reliably provide such a primary charge. Optimal charging devices would achieve such primary charge regardless of environmental variations, such as humidity and barometric pressure changes that affect the rate of ion generation and transport, and regardless of variation in the current energizing the corona discharge device, variation caused by aging or uncleanness of the corona electrodes, etc. Also, it is often desirable for such charging devices to reach the nominal primary charge level rapidly, for charging time can be the limiting parameter for the copy speed of the entire electrophotographic machine.
Grid-controlled charging devices (in which a grid located between the corona discharge electrode and the photoconductor is DC-biased to the surface potential desired for the photoconductor) have been very successful in achieving adequate primary charging in certain applications, e.g. where the photoconductor is moving past the charger during charging. However, in an in-place charging mode (where the photoconductor is stationary relative to the charger), the grid controlled charger is relatively slow. Also, in applications, e.g. where it is advantageous to charge and expose the photoconductor at the same location, the control grid presents optical problems.
For in-place charging of photoconductors it has been found advantageous to use DC-biased AC charging devices of the kind in which the level of DC biasing establishes an equilibrium potential for charging of the photoconductor surface (see e.g. U.S. Pat. Nos. 3,076,092 and 3,942,080). More specifically, in AC charging devices the net charge migration that occurs during an AC energization cycle constitutes the charge applied to the photoconductor surface during the cycle. By DC-biasing the AC energizing source to a predetermined positive or negative potential level, a preponderance positive or negative charge can be caused to migrate to the surface until the surface potential is sufficiently equal to the bias potential to create an equilibrium condition. Although these prior art devices provide a useful degree of control on the final charge level of the photoconductor, they remain sensitive to the environmental and other system variations noted above. Like the grid controlled charging devices they are relatively slow in attaining the final equilibrium charge.